Some computing devices, such as notebook computers, personal digital assistants, mobile communication devices (e.g., such as smartphones, cellphones, tablets, etc.), and others, may include user interface devices configured to use fingerprint sensors to acquire a fingerprint image from a user and, typically, to perform some operation or action based on the acquired fingerprint image. One type of such user interface device is a touch input device that operates a fingerprint sensor by way of capacitance sensing. For example, a touch input device may include a capacitive fingerprint sensor comprising an array of capacitive sensor elements. Capacitive sensing typically involves scan operations that periodically measure changes in capacitance associated with the capacitive sensor elements to determine presence, position, and/or other characteristics of a conductive object (e.g., such as a user's finger, stylus, etc.) relative to the sensing surface of a capacitive sensor.
The structure and operation of capacitive fingerprint sensors differ from other, common capacitive sensors (e.g., such as touch-screen sensors) in at least several aspects. For example, the active area of a capacitive fingerprint sensor is one to two orders of magnitude (e.g., typically about 100 times) smaller than the active area of a typical capacitive touch-screen sensor, and the number of sensor elements in a capacitive fingerprint sensor is about an order of magnitude less than the number of sensor elements in a capacitive touch-screen sensor. Further, the strength of a fingerprint signal acquired by a capacitive fingerprint sensor is two to three orders of magnitudes smaller than a signal acquired by a typical capacitive touch-screen sensor, thereby requiring extremely sensitive and low-noise circuitry in order to capture a usable fingerprint image. Thus, acquiring and processing fingerprint image data from capacitive fingerprint sensors poses some unique challenges that are not necessarily (if at all) addressed by techniques developed for typical capacitive sensors (e.g., such as touch-screen sensors).